1. Field of the Invention
This invention relates to a hand held optical reading device for reading indicia on the surface of a data carrier substrate, particularly to a hand held optical reading device that facilitates the focussing of the optical device relative to a small 2D target.
2. Description of the Prior Art
Two dimensional 2D bar code symbols are now common in the art. Symbologies such as PDF417 are characterized by stacked rows of data that in aggregate contain encoded information. Usually redundant information is included for error detection and correction. PDF417 is described in Wang, U.S. Pat. No. 5,243,655. Other two-dimensional symbologies are known. For example, Allais, U.S. Pat. No. 4,794,239 discusses stacked linear bar codes, and proposes a multi track symbology. Other 2D bar code symbols use non-stacked or matrix symbologies. An example of such a matrix symbology is described in U.S. Ser. No. 08/441,446, filed May 15, 1995.
Known bar code reading devices are capable of reading symbols even when the scanning axis of the reader is not rotationally aligned with the rows and columns. In the case of PDF417, this can be accomplished by "scan stitching", which utilizes positional information that is encoded in each row to identify a given row being scanned. Another approach, described in European Patent Application EP 0 385 478, involves transferring the optical pattern into a memory, and then evaluating the contents of the memory to determine the angular orientation of the symbol. Nevertheless it is more efficient to read the symbol in alignment with the active optical elements in the scanning device.
It is desirable to operate portable bar code scanners with light sources that consume low power, such as light emitting diodes. These devices have carefully optimized optical systems for creating an illuminating beam with sufficient angular divergence to read a bar code symbol that is wider than the optical window of the scanner, yet limit off-angle falloff of illumination to acceptable limits. Such scanners cannot be positioned too far from the symbol and still read effectively. Also, they often do not tolerate deviation of the scanner's optical axis from a normal to the surface of the target, because the working depth of field is intentionally kept small, in order to take advantage of the limited illumination available.
Two dimensional bar code symbols and other indicia on parts and equipment are becoming increasingly prevalent. The applications include automotive mechanical component housing and electronic component packaging. The smaller ones of these indicia range in size from 1/4" to as small or smaller than 1/32" in any one dimension.
Existing hand held indicia optical readers, including bar code readers, are not well suited for reading small indicia. One reason is that small indicia often appear on mechanical and electronic components which do not provide ready access to hand held devices. Another reason is that existing optical readers are wide angle devices and provide inadequate resolution of small marks or high density indicia on their imagers. The optical systems of these optical readers do not allow these optical readers to simultaneously both resolve small indicia and focus small indicia.
Optical reading devices that can resolve small marks (or indicia) have very shallow depth of field lens systems. These optical reading devices that can resolve small marks have both very shallow relative and absolute depth of field in relation to the depth of field of wide angle two dimensional bar code scanners reading standard bar codes. Optical reading devices that can resolve small marks have a small permissible range of distance between their lens system and the target for the target to be in focus.
Some bar code scanners are wand like devices that can be placed in close proximity to the target, and can resolve small marks. These scanners, however, read only linear one dimensional bar codes, because of their extreme proximity to the target. They are pixel readers, and are inadequate for two dimensional indicia.
Some bar code scanners have aligning members designed to help in aligning bar code at a pre-determined angle or designed to help in aligning the target in the field of view of the optical system, e.g. Sakai, U.S. Pat. No. 4,210,802. Sakai, U.S. Pat. No. 4,210,802, which discloses a scanner in which two parallel guide rails appended to the front of a housing are proposed to define a scanning region for the image sensor. These scanners are wide angle devices with large depth of field ranges and alignment systems that assume that the alignment device contacts the underlying substrate on the same plane as the indicia. The Sakai device is specifically designed to align 1D bar code in a plane perpendicular to the optical axis. 1D imagers require that bar code scanners are positioned on a perpendicular fixed orientation with respect to the plane of the one dimension bars, and parallel to the plane of the target, and not rotated freely relative to their indicia. These apparatus do not specifically address the problem of focussing on small 2D indicia, particularly a small 2D indicia target located on a relatively inaccessible component part within a device.